1. Technical Field
This invention relates generally to an apparatus and method for transmitting "masked" stereo audio signals in a signal distribution system such as a cable TV network. More particularly, the invention prevents the unauthorized reception of narrowband signals while minimizing bandwidth requirements needed for masking such signals.
2. Related Information
FIG. 1 shows the frequency spectrum of a conventional BTSC (Broadcast Television Standards Committee) encoded audio signal. Stereo signals consist of a left and a right speaker component, each of which is band-limited between 50 Hz and 14.5 Khz. The BTSC signal transmits the sum of the two components (L+R) 101 at baseband. The difference of the components (L-R) is amplitude modulated onto a suppressed carrier signal at twice the horizontal line frequency (2f.sub.h, where the line frequency is 15.73 KHz) thus producing two sidebands 103 and 104. The L-R signal is usually DBX encoded before AM modulation in order to provide greater immunity to noise introduced by FM demodulation at the receiver. A pilot tone signal 102, which is phase-locked to the carrier signal, is transmitted at the horizontal line frequency (f.sub.h).
At the receiving end, the L-R signal is AM demodulated and recombined with the L+R signal to generate the L and R audio signals. Conventional BTSC audio decoders lock on to the pilot tone 102 and re-generate the suppressed carrier signal; this pilot tone also indicates the presence of a stereo signal to BTSC decoders. In the absence of the pilot tone, BTSC decoders assume that only a monaural signal is transmitted.
It is often desirable in a cable television system to scramble or otherwise render unintelligible portions of the transmitted signal, including the audio signal, in order to prevent unauthorized reception. Various techniques for providing secure access to the audio portion of the transmitted signal have been devised. As one example, U.S. Pat. No. 4,956,862 to Robbins et al. describes a technique wherein the audio portion of the television signal is modulated at a frequency that is offset from the standard carrier frequency (e.g., 4.75 MHZ instead of 4.5 MHZ). Unfortunately, this technique has the disadvantage of increasing the amount of bandwidth required to carry the signal, and suffers from other side effects as well.
Other techniques, such as those exemplified by U.S. Pat. No. 5,159,631 to Quan et al., vary the frequency of the modulation carrier in a pseudo random fashion or use other means of scrambling the audio signal. Other methods make use of digital encryption techniques. However, such conventional approaches undesirably increase the overall bandwidth required to transmit the audio signal and require complicated circuitry. Accordingly, conventional techniques have been found to be unsatisfactory.